The missing starship Exeter is spotted in orbit of an inhabited planet. Kirk, Spock and McCoy board the Exeter, finding only the remains of the crew, wiped out by a disease which likely affects the boarding party now. Transporting to the planet, Kirk finds that Captain Tracey of the Exeter escaped his crew's fate, and the atmosphere on the planet is capable of eliminating the disease from the Enterprise landing party's bloodstreams. But more problems arise as Tracey discards his loyalty to the prime directive in an attempt to gain power in the planet's government.
